The state-resolved collision-energy dependences of Penning ionization cross sections σ(E) were measured in an energy range (60<E<400 meV) for N2 and CO2 with He*23S by using a very high-intensity He* beam (1.8×1015 s−1 sr−1) and detecting energy-analyzed electrons as functions of time-of-flight of He*. The partial ionization cross sections for Π states (N+2B:A2Πu, CO+2B:X2Πg, A2Πu) were observed to increase more rapidly with the increase of the collision energy than those for Σ states (N+2B:X2Σ+g, B2Σ+u, CO+2B:B2Σ+u,C2Σ+g). In the studied energy range, the repulsive walls for end-on collisions were indicated to be harder than those for side-on collisions. The directional peculiarity of the potential surfaces was related to the anisotropy in the hybridization of He* orbitals interacting with the target molecules.

Penning ionization of (CH3)4C and (CH3)3CCl upon collision with He*(2 3S) metastable atoms were studied by angular and collision-energy resolved Penning ionization electron spectroscopy. The observed Penning ionization electron spectra (PIES) revealed peculiar features for the totally symmetric ionized state of the target molecules which can be ascribed to ionization from a molecular orbital having carbon 2s atomic orbital characters; (1) the relative PIES intensity is extremely enhanced, (2) the intensity distribution of the vibrational fine structure in PIES is clearly different from that observed for ultraviolet photoelectron spectra, (3) the angular distribution of Penning electrons is highly isotropic, and (4) the collision energy dependence of the partial Penning ionization cross section is slightly increasing and rather flattened with respect to the increase of the collision energy. These features were found to be due to the formation of an excimer-like state involving C2s hole characters in the target molecule which induces Auger-like transitions selectively from orbitals having the C2s characters. Effects of attractive interaction potentials were observed for (CH3)3CCl on the ionized state ascribed to the Cl nonbonding orbital.

The kinetic-energy dependences of partial cross section σ(E) for the collisional ionization of H2O, H2S, O2, and Ar with He(2 3S) metastable atoms are measured in the energy range of 40–300 meV by collision-energy-resolved electron spectroscopy. The σ(E) functions are markedly dependent on the system and, in some cases, on final ionic states because of the anisotropy of incoming interaction potential surfaces or potential surface crossings with other potentials. Negative energy dependences of σ(E) are obtained for H2O+ (X 2B1, A 2A1, B 2B2) and H2S+ (X 2B1, A 2A1, B 2B2), whereas positive energy dependences for O+2 (a 4Πu, b 4Σ−g) and Ar+ (2P3/2). The σ(E) curves for O+2 (X 2Πg, A 2Πu) produced by Penning ionization via the ion-pair potential of He+(2S)+O−2(2Πg) are almost independent of the collision energy. The results are interpreted in terms of the semiclassical scattering theory of Penning ionization with assuming appropriate function forms to the potential and the transition rate.